1. Field of the Invention
The invention relates to a DC-DC converter, and a vehicular lamp provided with a lighting circuit that includes a DC-DC converter.
2. Description of Related Art
Various methods and circuit structures for a DC-DC converter for obtaining output voltage suitable for a load by stepping up or stepping down input voltage from a direct-current power supply are known. A structure that has a circuit portion as a switching regulator and a control circuit portion that performs switching control thereof is widely used as such a DC-DC converter. In some cases, a plurality of these kinds of DC-DC converters are arranged on a single circuit board. For example, a lighting circuit structure that generates with a DC-DC converter driving voltage depending on a light source such as a Light Emitting Diode (LED) and supplies current to the light source is used as a vehicular lamp in which a semiconductor light-emitting device such as the LED serves as the light source. Here, when lighting circuits of a plurality of systems are provided, such as a lighting circuit for a high beam LED and a lighting circuit for a low beam LED, for example, a plurality of the DC-DC converters themselves are also provided. In Japanese Patent Application Publication No. 2008-198915 (JP 2008-198915 A), a DC-DC converter (switching regulator) is provided for each of a plurality of light source units.
When a plurality of DC-DC converters are provided in a vehicular lamp or the like, a structure in which these circuits are arranged on a single circuit board is preferable to improve manufacturing efficiency and reduce the size of the overall apparatus. However, when a plurality of DC-DC converters are driven simultaneously, noise generated by the switching circuit of one DC-DC converter may affect the control circuit portion of another DC-DC converter and cause a malfunction. Typically, a circuit element such as a choke coil (a wound element), a transistor (switch element), or a diode (a rectifier element) that form the switching regulator is the noise source. For example, when the switching regulator is driven at a switching frequency of approximately several hundred KHz, a harmonic (for example, several tens to several hundred MHz) is generated by these circuit elements, and this harmonic affects the operation of an IC of the control circuit portion as noise. Also, the emission level of the harmonic noise generated by a plurality of DC-DC converters becomes high, which may also adversely affect other circuits.